1. Field of the Invention
The present invention relates to an image forming optical system, in particular to an optical system that can suitably applied to interchangeable lenses for interchangeable-lens cameras. The present invention also relates to an electronic image pickup apparatus equipped with such an image forming optical system.
2. Description of the Related Art
With increases in the number of pixels of image pickup elements and development of digital image processing techniques in recent years, digital cameras have replaced 35 mm film cameras. Moreover, with increases in the number of pixels of small size liquid crystal display panels used as viewfinders, the shift from single lens reflex cameras to new concept cameras without a quick-return mirror have been progressing.
Cameras without a quick-return mirror allow the design of small size, high performance image forming optical systems with a relatively short back focus. In addition, cameras without a quick-return mirror eliminate the dependency of the size of the viewfinder image on the imaging size of the lens or the diameter of the image circle. Therefore, a large viewfinder image can be obtained even if the image pickup format size is small. In addition, it is possible to design image forming optical systems that are small in size while having high performance.
On the other hand, small size cameras or compact cameras having a fixed lens, which have a short back focus and small image pickup format size, have been widespread. Zoom optical systems that are suitable for use in this type of cameras are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2002-48975, 2002-365545, and 2003-140043 as embodiments.
The optical systems disclosed in these documents have three lens units including, in order from the object side, a negative lens unit, a positive lens unit, and a positive lens unit. This zoom lens configuration is achieved by adding a positive third lens unit on the image side of the most basic two unit zoom lens configuration (composed of two lens units including a negative lens unit and a positive lens unit). The addition of the positive third lens unit enables a reduction in the overall length of the zoom lens at the wide angle end (at which the overall length tends to be large) and facilitates an increase in the zoom ratio.
A further increase in the number of lens units is not so effective, though zoom optical systems for what is called compact cameras having four lens units including in order from the object side a negative lens unit, a positive lens unit, a negative lens unit, and a positive lens unit have been developed, as described in Japanese Patent Application Laid-Open Nos. 2003-131130 and 2008-233611.
Eventually, the three unit configuration has become the most typical configuration.
It is necessary for optical systems for interchangeable-lens cameras to have a somewhat long back focus, because the total thickness of the lens system is more important than the overall optical length, and because there is a limitation on the allowable amount of extension of the optical system beyond the flange face of the camera on which the interchangeable lens is mounted.
On the other hand, interchangeable-lens cameras are required to produce images of higher image quality than images produced by what is called compact cameras. In view of this, interchangeable-lens cameras need to use a format having a relatively large size so as to be able to contain a sufficiently large number of pixels.
The pixel size in solid state image pickup elements in these years is smaller than the diameter of silver halide particles of photographic films. Therefore, satisfactory image qualities can be achieved even with formats smaller in size than the 135 format and the APS-C format.